Trends in semiconductor-based electronics industries suggest that future displays, solar cells, and electronic products will be made on flexible/conformal substrates. This transition is seen as inevitable to service the ever-present need and desire to reduce the size, weight, and cost of the products we use without sacrificing their performance. A wide gamut of devices, from displays, electronics, solar cells, and sensors, to name a few, would benefit from methodologies that result in the mass production of ruggedized, light weight, portable, small form factor, less power hungry, and lower cost products. Furthermore, new and novel markets and opportunities could be addressed and opened up if these devices could be made flexible and/or conformal.
Future generation semiconductor technologies for flexible/conformal substrates will most likely utilize organic semiconductors, rather than the inorganic semiconductors that have largely dominated semiconductor device manufacturing into at least the early 2000s. Organic semiconductors typically require much lower processing temperatures than inorganic semiconductors, allowing them to be used with flexible substrates that cannot withstand the high-temperature processes used for inorganic materials. Inorganic semiconductors can be processed at lower temperatures. However, low temperature deposition of inorganic semiconductors used in industry typically yields amorphous and polycrystalline phases of the semiconductor, and these phases tend to have significantly lower carrier mobilities than can be realized in single-crystal variants of the same semiconductor.
For example, hydrogenated amorphous silicon is a ubiquitous material used in the liquid crystal display (LCD) industry and the emerging solar cell industry. However, electron carrier mobility in hydrogenated amorphous silicon is typically in the range of 0.3 cm2/Vs to 1.2 cm2/Vs, depending on deposition conditions. Amorphous silicon can be re-crystallized, post deposition, using a technique called “solid-phase crystallization.” The resulting nano- and micro-grained polycrystalline material might have mobilities between 10 cm2/Vs to 250 cm2/Vs. However, this is still significantly below single crystal silicon mobilities of >450 cm2/Vs. A similar pattern of deteriorating electrical performance is found in most semiconductors as the material goes from the single-crystal to the poly-crystal to the amorphous phase.
The performance of some p-n-junction-based devices, such as solar cells, is based in part on the width of the depletion region within the device. For example, for an ideal solar cell, higher performance at longer wavelengths requires the width of the depletion region to be as wide as possible in order to obtain high photocurrent. This is due to the fact that typically the absorption coefficients of semiconductors are lower at higher wavelengths when compared to lower wavelengths. A wider depletion region can be achieved by reducing the dopant concentration. However, lower doping reduces the open-circuit voltage, Voc, of the solar cell. At lower wavelengths, the absorption coefficients tend to be high, and, therefore, the width of the depletion region must be made as small as possible so that the photons will be absorbed near the junction. However, a smaller depletion region will increase unwanted surface recombination thus the series resistance will increase. Consequently, the solar cell's efficiency is reduced. This means the solar cell efficiency is limited by the device and design parameters.